The coating of surfaces of ferrous-containing materials, such as, for example, steel and other iron alloys, with copper is well known. This coating may be obtained by electroplating copper from a solution containing cupric ions or by contacting the surface of the ferrous-containing material with an aqueous acidic solution of cupric ions. The latter method, which involves the displacement on the surface of the material of the iron by copper in accordance with the equation EQU Fe+Cu.sup.++ .fwdarw.Cu+Fe.sup.++,
does not require the use of an electric current and is simpler to perform.
While apparently simple to carry out, the coating of copper on surfaces of ferrous-containing materials by the non-electrolytic method does present many problems. The reaction itself is relatively rapid, and the copper coating from such rapid reaction is very porous, even mossy, and does not adhere well to the surface. It thus becomes necessary to slow down the rate of reaction by the use of an inhibitor. This inhibitor is added to the aqueous acidic solution containing the cupric ions. By proper control of the rate of the reaction there can be obtained a dense, adherent deposit of copper on the surface of the ferrous-containing material.
A large number of inhibitors are available for this purpose and examples of such inhibitors are disclosed in U.S. Pat. Nos. 2,410,844 and 3,535,129; German Pat. No. 714,437; British Pat. No. 927,576 and French Pat. No. 1,257,758.
It has also been found useful, as disclosed in U.S. Pat. No. 3,535,129, to include surfactants and organic dyes in these coating solutions. The surfactants presumably aid in promoting the contact of the coating solution with the surface of the ferrous-containing material, while the dyes provide added clarity to the coating solution and brighten the appearance of the copper coating. In addition, the dyes may also provide some inhibition of the rate of reaction. However, although these additions do improve the appearance, thickness and adherence of the copper coatings, such improvements tend to apply only to the initial coating--for as the reaction proceeds the later coatings become poorer in quality. This is probably due to the accumulation of ferrous ions and other by-product ions (e.g. Cl.sup.-, SO.sub.4.sup.=) in the coating solution from the above reaction.